Theory

Bulking of Fine Aggregate (sand) is a phenomenon in which the volume of sand increases when some amount of moisture is added to it. It depends upon the particle type, size and the amount of water content. When fine aggregates are damp or contain excess moisture, they tend to adhere to each other and increase in volume. This phenomenon can lead to inaccurate measurements and proportions in concrete mixtures if not accounted for. Finer is the sand, more will be the bulkage.

The extent of bulking can vary depending on factors like the grading of the sand and the amount of moisture present. Maximum bulking occurs at moisture content of 4-6%, beyond which the bulking effect gradually decreases. When sand becomes completely saturated (inundated with water), the volume returns to its original dry volume as water fills all the voids between particles. This principle forms the basis of bulking determination tests.

Practical Application

To account for bulking in construction practices, engineers and concrete mix designers often adjust the proportions of fine aggregate in the mix based on its moisture content. This adjustment ensures that the correct volume of dry sand is used in the concrete mixture, preventing potential issues such as reduced concrete strength and workability. The measured volume of damp sand must be increased by the percentage bulking value to obtain the correct quantity of sand as per the nominal mix design (based on dry sand).

Formula for Bulking

The formula for calculating the bulking of sand is:

1. Percentage bulking of sand:

Percentage bulking of sand = [(H - H') / H'] × 100

Where:

• H = height of loose damp sand (cm)
• H' = height of saturated/inundated sand (cm)

2. Alternative Formula (Measuring Cylinder Method):

Percentage bulking = [(200 - y) / y] × 100

Where:

• 200 = initial volume of loose sand in cylinder (ml)
• y = final volume of saturated sand after adding water (ml)